Spinal implantation methods utilizing a middle expandable implant

ABSTRACT

Artificial disk implant and methods for implanting same, the implant having a member (32, 34, 36, 77, 92, 94) for adapting in size and shape to the anatomical space between vertebrae, and apparatus (25, 42, 60, 112) for expanding the implant in the middle portion thereof to conform to the space. In one embodiment, there is provided an artificial intervertebral disk implant having a cylindrical body (20, 41, 56, 88) comprised of cylindrical subunits (32, 34, 36, 92, 94) capable of expansion. In another embodiment, rectangular members (34, 36) or elongate ribs (77) capable of expansion are provided. The implant can be used alone or in various combinations for the purpose of spinal fusion.

This application is a continuation-in-part of my application Ser. No.08/106,148, filed on Aug. 13, 1993, now abandoned. Ser. No. 08/106,148was filed as a continuation application of application Ser. No.07/786,758, which was filed on Nov. 1, 1991 and is now abandoned, Ser.No. 07/786,758 having been itself filed as a continuation-in-part ofapplication Ser. No. 07/659,758, filed on Feb. 22, 1991, now issued asU.S. Pat. No. 5,171,278.

BACKGROUND OF THE INVENTION

This invention relates to an intervertebral disk implant and a method ofimplanting same. More specifically, the present invention relates tocylindrical and rectangular disk implants which are expandable in themiddle portion which are used alone or in various combinations for thepurpose of spinal fusion.

The spine is a flexible structure comprised of thirty-three vertebraeseparated and cushioned from each other by fibrous intervertebral disks.If the spine is injured or becomes diseased, surgical interventioninvolving removal of one or more disks, and fusion of the adjacentvertebrae, may be indicated. The more frequent injuries are in the lowerlumbar and in the lower cervical regions.

Treatment of a herniated disk in the neck and in the lumbar regioncontinues to be a challenging field of medicine. The classical treatmentfor a ruptured disk continues to be diskectomy, i.e., removal of thedisk from between the vertebrae. In this process, all or a portion ofthe intervertebral disk is removed, leaving a defect which continues tobother the patients throughout the rest of their lives. An additionalprocedure is to replace the disk space with a bone graft, usually bonechips cut from the patient's iliac crest, bringing about fusion of thevertebrae above and below the disk, eliminating the empty space betweenthe vertebrae.

Theoretically, a diskectomy with fusion is a satisfactory procedure,though not ideal because the replaced bone does not have any of thefunctions of the cartilage tissue of the disk, i.e. no cushioningeffect, and has complications because of several factors. First, thebone plug used to pack the disk space does not conform to the shape ofthe disk because the disk bulges maximally in the center. The disk spaceis wider in the middle and narrower at its anterior and posterior ends.Consequently, a bone plug having its maximum width at the center, e.g.,one which is shaped to fit the space, cannot be inserted through thenarrow mouth of the disk space. For this reason, the various bone plugswhich are currently available commercially have only four contactpoints, i.e. at the front and back of the disk space. Secondly, accessto the disk is from one side or the other of the dorsal spine of theadjacent vertebrae, leaving a space that is "off-center" relative to thebodies of the adjacent vertebrae. An implant inserted into thatoff-center space, therefore, replaces only a portion of the disk andconsequently contacts only a portion of the bodies of the adjacentvertebrae such that the stability of the implant is even moreproblematical than might be apparent from the limited contact resultingfrom the shape of the intervertebral space in the first place. Anothercomplication is the possibility of infection or other conditions whichmay require the removal of the implant. Also, if the bone pieces do notfuse, they may eventually extrude out of the disk space, causingpressure on the nerve roots.

Various prosthetic disk plugs, or implants, are disclosed in the art,but all are characterized by limitations of not conforming to the shapeof the disk space, lack of stability when inserted off-center, inabilityto be removed, or other disadvantages. For instance, U.S. Pat. No.4,863,476 describes an elongated body divided longitudinally into twoportions having a cam device movable therebetween for increasing thespace between the two body portions. However, that device is generallycylindrical in shape such that the only contact points are at the frontand back of the disk space, creating increased likelihood of instabilityand generally rendering that device unsuitable for use after partialdiskectomy. The art also discloses intervertebral disk prostheses (e.g.,U.S. Pat. Nos. 3,867,728, 4,309,777, 4,863,477 and 4,932,969 and FrenchPatent Application No. 8816184) which may have more general contact withthe adjacent disks, but which are not intended for use in fusion of thedisks. The art also includes spinal joint prostheses such as isdescribed in U.S. Pat. No. 4,759,769, which is again not indicated foruse when fusion is the preferred surgical intervention.

From this prior art, it is apparent that there has long been a need fora disk plug, or implant, capable of supporting the disk space after asimple diskectomy for fusion of adjacent vertebrae, and the object ofthe present invention is to provide such an implant.

SUMMARY OF THE INVENTION

An intervertebral disk implant is described for implantation into thedisk space after surgical removal of all or a portion of a diseased ordamaged disk. Implants according to this invention include means forchanging the shape of the implant to adapt to the shape of the diskspace by expanding the implant to conform to the contour of that space,and are, for that reason, referred to herein as being "middleexpandable".

In one embodiment, there is provided an intervertebral disk implant witha cylindrical body comprised of subunits capable of radially outwardexpansion. In another embodiment, there is provided an implant having asubstantially rectangular body likewise comprised of subunits capable ofradially outward expansion. Both are disk plugs expandable in the middleportion to provide contact with substantially the entire area of thedisk space against the vertebral bodies.

In the method of the present invention, there is provided a method offusing two adjacent vertebrae after removal of all or a portion of thedisk from therebetween which comprises inserting a disk implant into thespace from which the disk has been removed, expanding the middle portionof the implant outwardly in a radial direction, injecting cancellousbone chips into the disk space medial to the implant, and applying aphysiologically compatible adhesive over the bone chips medial to theimplant to close off the opening of the disk space.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, FIG. 1 is a projected view of one embodiment of thedisk implant of the present invention.

FIG. 2 is a cross sectional view of the disk implant of FIG. 1 takenalong the line 2--2 in FIG. 1.

FIG. 3 is a projected view of the central axis of the disk implant ofFIG. 1 having the members coiled therearound removed therefrom.

FIG. 4 is a projected view of the implant of FIG. 1 after expansion ofthe middle portion thereof.

FIG. 5 is a projected, exploded view of a second embodiment of the diskimplant of the present invention.

FIG. 6 is a projected view of the implant of FIG. 5 showing that implantafter expansion thereof.

FIG. 7 is a top, plan view of a lumbar vertebra of a human patienthaving a top, plan view of the implant of FIG. 6 superimposed thereon toshow the spatial relationship of the implant to the adjacent vertebraeafter insertion into the disk space.

FIG. 8 is a projected view of another embodiment of the implant of thepresent invention.

FIG. 9 is a projected view of the disk implant of FIG. 8 after expansionof the middle portion thereof.

FIG. 10 is an exploded, projected view of a fourth embodiment of theimplant of the present invention.

FIG. 10A is a side view of two hinged members comprising the middleportion of the implant of FIG. 10 and removed therefrom.

FIG. 11 is a projected view of a fifth embodiment of the disk implant ofthe present application.

FIG. 12 is a cross sectional view of the disk implant of FIG. 11 takenalong the line 12--12 in FIG. 11.

FIG. 13 is a side view of the disk implant of FIG. 11 showing a portionbroken away therefrom.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a cylindrical embodiment of the disk implant of thepresent invention. The disk implant 20 shown in that figure is comprisedof a strong, thin nonporous material. Suitable materials for the diskimplant 20 include modified carbon, titanium, steel, metals and/or metalalloys having a memory (see below), physiologically inert and/ormedically compatible polymers such as a urethane or DELRIN® polymer, orany generally rigid, biologically compatible material used for surgicalimplants. It is also useful to use a material which is compatible withmagnetic resonance imaging (MRI) procedures. The disk implant 20 iscomprised of a plurality of longitudinally aligned sections, or subunits22, 24 and 26, and a screw 28 to which each section is mounted (asdescribed below) is turned to cause differential, radially outwardexpansion of subunits 24 and 26. The subunits 24 and 26 are preferablycomprised of a material capable of maintaining spring tension and aremounted to and wound around an elongate longitudinal axis in the form ofcentral rod 25 (see FIGS. 2 and 3) integral with screwhead 28. Becauseof this structure, each of the subunits is conveniently referred to asincluding a coiled member as identified at reference numeral 32.

Each coiled member 32 is mounted to central rod 25 by welding, riveting,or by other manner depending upon the material(s) comprising the sheet32 and central rod 25 as known in the art. In the preferred embodimentshown in FIGS. 1-4, the central rod 25 is provided with a flat 23 toprovide a stable surface for mounting of the member 32 thereto by, forinstance, welding. At the other, free end of each coiled member 32, thecoiled member 32 is beveled as at reference numeral 33 so as to providea smooth, generally round exterior surface on each of the subunits 24and 26 and to facilitate the sliding of the free end of coiled member 32along the outside surface thereof as the subunits 24 and 26 are expandedradially outwardly as described below.

A Phillips head-type slot 18 is provided in the screwhead 28 forrotation of the rod 25 as described below, and the head 28 is providedwith a plurality of teeth 19 for interdigitating with the reciprocalcavities in the lock nut 21 to prevent undesired rotation of central rod25. The Allen screws 30 are loosened to force lock nut 21 away from theend surface 27 of subunit 22 so that the teeth 19 on the head 28 ofcentral rod 25 are disengaged from the cavities in lock nut 21 to allowrotation of screwhead 28 and rod 25. Alternatively, either or both ofrod 25 or lock nut 21 is comprised of a resilient, medically compatiblepolymer material which allows rotation of the teeth 19 past the cavitiesin lock nut 21 in one direction but not the other. The expanded shape ofa section of the disk implant 20 is shown in FIG. 2.

Turning screwhead 28 and central rod 25 using the slot 18 expands thesections 24 and 26, which remain expanded due to the interaction of theteeth 19 and the cavities in lock nut 21 and the compression of theimplant 20 between the bodies of the vertebrae above and below theimplant 20 once inserted into the disk space. In other words, engagementof the free end of coiled member 32 by the adjacent vertebrae preventsthe slipping of the free end of the coiled member 32 around the outsidecircumference of implant 20 such that members 32 do not "re-wind" afterbeing expanded.

As shown in FIG. 3, central rod 25 is provided with a portion 29approximately mid-way between the ends thereof having a larger diameterthan the rest of the central rod 25. By use of the central rod withsections of different diameters and/or thicknesses of the cylindricallywound member 32, the subunits 24 and 26 are differentially expanded.Turning screw 28 allows for maximal expansion of the subunit 26 andmoderate expansion of the subunit 24 because the member 32 comprisingsubunit 26 is mounted to the rod 25 on the portion 29 of larger diameterwhile each of the members 32 comprising subunits 22 and 24 is mounted tocentral rod 25 between the portion 29 and the subunits 22. Turning thecentral rod 25 uncoils the members 32 because each member 32 is attachedto the central rod 25.

FIG. 4 illustrates the cylindrical disk implant 20 in its radiallyexpanded form. Once expanded, the implant cannot be removed from thedisk space except by turning the allen screws 30 to either back out orremove lock nut 21, thereby allowing rotation of rod 25.

Referring now to FIGS. 5 and 6, an alternative embodiment of the implant20 is shown at reference numeral 56. Implant 56 is comprised of a singlepiece of metal, such as a titanium alloy, or medical grade polymericplastic, such as DELRIN®, which is resilient and has a memory for theshape in which it is molded, shown in FIG. 6. Implant 56 is molded inthe same generally elongate, cylindrical shape as the implant 20 shownin FIGS. 1-4, but is molded in a shape in which the middle portion 58thereof is normally expanded radially outwardly from the central axis ofthe cylinder. An elongate screw 60 is provided having two sets ofthreads 62 and 66 thereon, the former for engaging the threads 68 formedin the bore 70 extending longitudinally through implant 56, the latterfor engaging a similarly formed set of threads located in the bore 70 atthe other end of implant 56 and therefore not visible in FIGS. 5 and 6.A slot 72 is formed in the head 74 of screw 60 for turning screw 60 tomove the opposite ends 76a and 76b of implant 56 away from each other,thereby extending implant 56 and decreasing the radially outwardexpansion of the middle 58 thereof as shown in FIG. 5 for insertion intothe disk space. Longitudinal slots 75 are molded into implant 56 to formribs 77 which flex to allow the extension and outward expansion ofimplant 56 in this manner.

As noted above, the instability of prior implants once inserted into thedisk space is problematical, and FIG. 7, showing the implant 56 in placerelative to the body 78 of an adjacent lumbar vertebra 80 illustrateshow the apparatus of the present invention overcomes this limitation ofprior implants. The implant 56 is inserted into the disk space in ananterior-posterior (A-P) orientation, the dorsal spine 82 of vertebra 80being pointed posterially. As clearly shown in FIG. 7, when sopositioned, implant 56 occupies only a portion of the surface area ofthe vertebral body 78, the remainder of the area being occupied by thatportion of the intervertebral disk (not shown) which is not removedduring the diskectomy procedure (or, in a fusion procedure, this area ispacked with cancellous bone chips). Access to that area is from theposterior aspect of the disk medial to the implant. In addition, theperiphery 88 of vertebral body 78 is, as described above, thicker thanthe central portion 90 of body 78, further limiting access and creatingan uneven surface on which the body 78 bears on the implant. However,because of the expansion of only the middle 58 of implant 56, theimplant 56 is stable in the A-P orientation shown. Once implanted, thescrew 60 is backed out of the bore 70 in implant 56 and implant 56assumes the shape shown in FIGS. 6 and 7.

FIG. 8 depicts a rectangular disk implant 31 constructed according tothe present invention. Turning Phillips head 39 of screw 42 encapsulatedin a sheath 44 (best shown in FIG. 9) formed in the hinged members 34and 36 forming intermediate subunits in the same manner as the subunits,or sections, 24 and 26 of implant 20 causes the radially outwardexpansion of superior hinged members 34 superiorly and inferior hingedmembers 36 inferiorly. Although shown in FIGS. 8 and 9 with two of thehinged members 34 and 36, it will be understood by those skilled in theart who have the benefit of this disclosure that the plug, or implant,31 may be provided with four, eight, or even more of the hinged members34 and 36 as shown at reference numerals 92 and 94 in FIG. 10 andnumeral 41 in FIGS. 11-13. The expanded shape of the rectangular diskplug 31 is illustrated in FIG. 9. Hinged members 34 and 36 are securedto an end cap or subunit 33 by hinge 38 and to each other by hinge 46.Upon rotation of screw 42 using a conventional screwdriver and thePhillips head slot 39, the end caps 33 are drawn closer together bymovement along the threads of screw 42. To insure that the members 34and 36 expand radially outwardly from screw 42, the ends 48 of eachrespective member 34 and 36 abutting the end caps 33 are angled so as tocreate a force vector outwardly away from screw 42 when end cap 33exerts pressure on the surface 48, the hinge 38 being mounted in theacute angle formed by surface 48 and end cap 33.

In one embodiment (best shown in FIGS. 11-13 and discussed below), thetendency of this force vector to cause the members 34 and 36 to expandis increased by angling the face 50 of one member 34 or 36 in the samedirection as the angle in the surface 48. The surface 52 of the opposedmember 34 and 36 is similarly angled, but with a bearing surface 54formed therein that is angled in the same direction as the angle insurface 48 and face 50 so that the face 50 rides upwardly onto bearingsurface 54 to translate the opposed, end-to-end force vectors applied toend caps 33 by rotation of screw 42 into a force vector having aradially outward (from screw 42) component. By referring to FIGS. 11-13,it can be seen that the radially outward expansion of the middle portionof implant 31 caused by rotation of the screw 42 effectively simulatesthe opening of two opposed umbrellas, and the particular embodimentshown in those figures may be conveniently referred to as having a"double umbrella" configuration.

A threaded lock nut 40 is inserted over Phillips screw head 39 (see FIG.8). Lock nut 40 prevents the members 34 and 36 from moving onceexpanded. Removing lock nut 40 provides access to screw head 39 to allowmembers 34 and 36 to return to the position shown in FIG. 8.

The above-referenced, double-umbrella configuration of the implant ofthe present invention is illustrated at reference numeral 88 in FIG. 10.In this embodiment, the hinged members 92 and 94 are mounted on pivotpins 96 to the first and second end members 90 and 98, respectively, aswell as to each other, most of the pins 96 and all but two sets of thehinged members 92 and 94 being omitted from the figure for purposes ofclarity. The pivot pins 96 which mount members 92 and 94 to the ends 90and 98 are received within the bores 100 and 102 formed in each endmember 90 and 98, the bores 100 and 102 being numbered separately todraw attention to their arrangement on the end members 90 and 98. Theears 104 on hinged members 92' and 94' are longer than the ears 106 onhinged members 92" and 94" and the bores 100 for receiving the pivot pin96 are located closer to the end surface 108 of end member 90 (and thecorresponding end surface of end member 98 at the opposite end ofimplant 88) than the bores 102. By this arrangement, the strength of theimplant 88 is significantly increased.

Expansion of the middle portion of implant 88 is accomplished by turningthe screw 112 using the hex head 114 formed at one end thereof, theother end of screw 112 being received by the threads 115 formed in thesecond end member 98. To increase the tendency of the hinged members 92and 94 to expand in the radially outward direction, the holes in thehinged members 92 and 94 in which pivot pins 96 reside are offset alongthe longitudinal axis of implant 88. The offset holes are better shownin FIG. 10A in which one pair of the members 92 and 94 is shown in sideview removed from implant 88. The direction of expansion is shown by thearrow 95 in FIG. 10A and, as can be seen, the center holes 97 are offsetoutwardly (e.g., in the direction of arrow 95) relative to the holes 99at the ends of hinged members 92 and 94 (e.g., in the ears 106).

A lock nut 116 having threads 118 formed in the outside surface thereofis received by the threads 120 formed in the bore 122 in end member 90through which the screw 112 is received for preventing undesiredrotation of screw 112. Lock nut 116 is provided with a hex slot 124 tofacilitate insertion and/or removal and hex slot 124 extends all the waythrough lock nut 116 and is of large enough size that a hex key can beinserted through slot 124 and into hex head 114 for turning screw 112without adjustment of lock nut 116.

Another embodiment of the double-umbrella configuration of the implantof the present invention is shown at reference numeral 41 in FIGS.11-13. As is the case with the implant 88 shown in FIG. 10, the implant41 is generally cylindrical in shape, yet utilizes the hinged member 34and 36 construction of implant 31 shown in FIGS. 8 and 9. FIG. 12 showsa projected view of the disk implant 41 shown in FIG. 11 having themembers 34 and 36 cut in section. This view shows how the hinged members34 and 36 fit together in the unexpanded position due to their beveledsides 64, giving the implant 41 its generally cylindrical shape. Thesides 110 of the hinged members 92 and 94 of implant 88 are similarlybeveled (FIG. 10).

All of the disk implants of the present invention are expandable in themiddle portion, i.e., the portion intermediate the ends, to contactsubstantially the entire anterior-posterior dimension of the disk spaceagainst the vertebral bodies as described above in connection with thedescription of FIG. 7. If a complete intervertebral fusion is beingperformed, the plug is used in conjunction with intervertebralcancellous bone packing. Because of the support provided by the plug,until fusion is established, the cancellous bone pieces have a betterchance of fusion due to the presence of the implant, and the bone piecesand the disk implant have a better chance of staying in theintervertebral disk space. Alternatively, the plug is used to maintainthe spacing between vertebrae and can be used in conjunction withintertransverse posterior lateral fusion. In short, the implant acts asa physiological support for the rest of the patient's life or until abone fusion is established.

The disk implant of the present invention may have additionalindications, e.g. short segment scoliosis, where the curvature of thespine can be corrected by distracting the vertebral bodies on the insideof the curvature. By expanding the middle portion of the plug inside thedisk space, the vertebral bodies are distracted, thereby helpingstraighten the spinal column.

If no bone graft is planned, diskectomy can be made minimally throughone side exposure so that when the disk plug is inserted and expanded,it will occupy the empty space. Because there is no further movement atthis disk space, the chance of recurrent disk herniation is minimized.Also, the likelihood of recurrent disk herniation due to opening andclosing of the space on the side of the diskectomy is reduced becausethe disk plug closes this mouth. Consequently, in addition to theadvantages of a one sided, simple diskectomy, the risk of recurrent diskherniation can be reduced.

The cylindrical 20, 41, 56, and 88 and rectangular 31 implants areinserted after a simple diskectomy. Ordinarily, the size of the diskimplant is approximately 2.5 to 3.5 centimeters in length and 1.0 to 1.5centimeters in height and width. The same plug in smaller dimensions isused in thoracic and cervical levels where indicated.

By reference to the figures, it can be seen that both the rectangularand the cylindrical implants have the common feature of being expandablein the middle without changing the diameter of the dimensions of the twoends. Consequently, surgery is performed as in simple diskectomy, andthe disk is exposed through a small laminotomy. The disk material isremoved and any nerve root compression is corrected. The posteriorlongitudinal ligament and disk cartilage are removed until the vertebralsurfaces are exposed above and below the disk space. The shape of thedisk space determines whether the disk plug used is cylindrical orrectangular. The disk plug is then inserted and hammered into place sothat the anterior end of the disk plug almost touches the anteriorlongitudinal ligament. Subsequently, using a Phillips screwdriver, theposterior screw end is turned. This implant method also gives gooddistraction to the vertebral bodies. In the case of simple diskproblems, no further treatment may be required.

When used in interbody fusion, cancellous bone chips are made into veryfine particles and pumped into the disk space medial to the disk plugand packed into the space. The posterior longitudinal ligament is intactto the opposite side and to the center of the disk space. Thesecancellous bone chips are held tightly in place. Since the mouth of thedisk space is closed with the disk plug, the risk of the cancellous bonechips coming out is minimized. Also, the disk plug prevents the openingand closing of the disk space, thus preventing the bone chips comingout. If necessary, a small amount of a physiologically compatibleadhesive of a type known in the art is applied over the cancellous bonechips just medial to the disk plug to close off the remaining portion ofthe opening of the disk space. The patient should be able to ambulatesoon after the surgery because of the stability given by the disk plug.Before narrowing of the disk space occurs, the cancellous bone chipswill have started the fusion process.

If a posterior lateral intertransverse fusion is desired, this procedureis also done in conjunction with the middle expandable disk plug. Thedisk plug is applied as explained above and the posterior lateral fusionperformed. Since the disk plug provides stability to the spine until theposterior lateral fusion is solid, the patient can ambulate soon afterthe surgery. This procedure also prevents the disk space narrowing,which is a common problem with posterior lateral fusion.

What is claimed is:
 1. A method of maintaining the space between twoadjacent vertebrae of a patient after removal of the disk fromtherebetween comprising the steps of:inserting an elongate implantcomprised of a central rod having two ends and an intermediate portionmounted thereto into the space between two vertebrae after removal ofthe disk therefrom, the implant having a length which approximates theanterior-posterior dimension of the body of the vertebrae and a verticaldimension small enough to allow the insertion of the implant; rotatingthe central rod to expand the intermediate portion of the implantradially outwardly to conform the shape of the implant to the shape ofthe anatomical region of the disk space into which the implant isinserted; and preventing the rotation of the central rod to preventreversal of the outward radial expansion of the intermediate portion ofthe implant.
 2. A method of claim 1 further comprising injectingcancellous bone chips into said disk space medial to the implant.
 3. Amethod for fusing two adjacent vertebrae after removal of a portion ofthe disk from therebetween comprising the steps of:inserting an elongateimplant through an opening into a space between two adjacent vertebraeof a patient after removal of the disk from between the vertebrae, theimplant having a length which approximates the anterior-posteriordimension of the body of the vertebrae and a vertical dimension smallenough to allow insertion of the implant; expanding the middle portionof the implant outwardly in a radial direction to conform the shape ofthe implant to the shape of the space from which the disk has beenremoved; injecting cancellous bone chips into the space between thevertebrae medial to the implant; and applying a physiologicallycompatible adhesive over the cancellous bone chips medial to the diskimplant to close off the opening into the space from which the disk hasbeen removed.
 4. The method of claim 3 wherein expansion is accomplishedby unwinding a plurality of spring-tensioned members comprising themiddle portion of the implant, each of the members being mounted on andwound around a common longitudinal axis.
 5. The method of claim 3wherein expansion is accomplished by rotating a central rod, causingfirst and second end caps having holes therethrough for receiving therod to move closer together to force hinged members comprising themiddle portion of the implant radially outwardly.
 6. The method of claim1 wherein expansion is accomplished by unwinding a plurality ofspring-tensioned members comprising the intermediate portion of theimplant, each of the members being mounted on and wound around a commonlongitudinal axis.
 7. The method of claim 1 wherein rotation of thecentral rod causes first and second end caps having holes therethroughfor receiving the central rod and comprising the ends of the implant tomove closer together to force hinged members comprising the intermediateportion of the implant radially outwardly.
 8. The method of claim 1further comprising injecting cancellous bone chips into the disk spacemedial to the implant.
 9. A method of maintaining the space between twoadjacent vertebrae of a patient after removal of a portion of the diskfrom therebetween comprising the steps of:inserting an elongate implantinto the space between two vertebrae after removal of a portion of thedisk therefrom, the implant having a length which approximates theanterior-posterior dimension of the body of the vertebrae and a verticaldimension small enough to allow the insertion of the implant; expandingthe implant radially outwardly in the portion intermediate the endsthereof by rotating a central rod, causing first and second end capshaving holes therethrough for receiving the rod to move closer togetherto force hinged members comprising the intermediate portion of theimplant radially outwardly, to conform the shape of the implant to theshape of the anatomical region of the disk space into which the implantis inserted; and preventing the reversal of the outward radial expansionof the intermediate portion of the implant.
 10. A method of stabilizingthe spine of a patient comprising the steps of:removing a portion of anintervertebral disk of the patient; inserting an elongate implant intothe space between the vertebrae from which the portion of theintervertebral disk has been removed and off center relative to thebodies of the adjacent vertebrae, the implant having a length whichapproximates the anterior-posterior dimension of the bodies of thevertebrae and a vertical dimension small enough to allow insertion ofthe implant through the gap between the vertebrae and adjacent thedorsal spine thereof; expanding the middle portion of the implantoutwardly in a radial direction to conform the shape of the implant tothe shape of the space from which the portion of the intervertebral diskwas removed; and injecting cancellous bone chips into the space betweenthe vertebrae and medial to the implant.
 11. The method of claim 10further comprising applying a physiologically compatible adhesive overthe cancellous bone chips to close off the gap through which the implantwas inserted.